A head mounted display (HMD) which is a display mounted on the head is known. The HMD generates image light representing an image by using, for example, a liquid crystal display and a light source, and guides the generated image light to a user's eyes by using a projection optical system or a light guiding plate, thereby allowing the user to visually recognize a virtual image.
PTL 1 discloses a surgical imaging system and a surgical robot using such a head mounted display. In the technique disclosed in PTL 1, a user (surgeon) of the head mounted display operates a surgical manipulator while visually recognizing, as a virtual image, an image with an endoscope which is inserted into a body cavity of a patient, thereby performing surgery. PTL 2 discloses a face fitting image display device for medical use (head mounted display). In the technique disclosed in PTL 2, when a user of the face fitting display device visually recognizes an endoscope image, the user directs his/her visual line in a horizontal direction and views an LCD disposed in front of a main body. In addition, when the user visually recognizes external scenery, the user directs his/her visual line in a vertical direction and views an observation window disposed under the main body.